In the field of NMR spectroscopy, a sample is surrounded by an NMR probe that consists of a radio frequency (RF) coil tuned to generate a field at a desired excitation frequency and receive a return NMR signal. More complex probes will generate multiple frequencies so as to excite the nuclei of more than one different element in the sample (e.g., hydrogen nuclei 1H (proton) and fluorine nuclei 19F). These “double resonance” probes (in the case of a probe generating two separate frequencies) and “triple resonance” probes (in the case of a probe generating three separate frequencies) have been used for many years, with varying degrees of success. One of the problems faced by multiple resonance probes arises when trying to adjust the response at one frequency without disturbing that of another.
In systems having a single sample coil, it is necessary to generate each of the desired resonant frequencies and apply them to the coil, and some form of frequency isolation is incorporated into the circuits themselves. Transmission line resonators have been used to produce high Q resonances with high power handling for NMR probes, particularly at high Larmor frequencies such as 1H and 19F. These resonators have nodes at which the electric field is at a minimum, and at these locations circuitry for lower nuclei resonances can be added without affecting the high frequency resonances. This allows a single sample coil to be used to excite an NMR sample at several isolated frequencies, as opposed to using double orthogonal coils to prevent mutual coupling between the resonances. A single sample coil has the advantage of improved sensitivity with higher filling factor and better power handling without inter-coil arcing. However, use of a single sample coil also has the disadvantage of efficiency tradeoffs between the high frequency and low frequency channels.
In existing systems, the resonant frequency of the sample coil determines the trade off between the efficiency of the high frequency (such as 1H) and low frequency (such as 13C or 15N) channels. Increasing the self resonance of the sample coil, either by reducing its inductance or capacitance, improves 1H efficiency while degrading the efficiency of a lower frequency. Decreasing the self-resonance of the sample coil has the opposite effect.